Sounds a bit like a question with an obvious answer... But I'm asking it just to make sure. (I apologize if my question was posted in the wrong section, I don't use forums that often.)
Well, the point of this is to send a "low power" audio signal (either modulated, or a raw audio signal.. depending on what's easier to implement) AND around +24VDC over a single pair of wires, for the "active speakers" to amplify them, instead of having a larger central amplifier.

I've read into some articles but they're for digital signals...
For context, this is for a "voice evacuation system" for a hobby fire alarm system. I wouldn't need anything crazy as I don't think I'd need much power being sent over these lines.

Some info:
40-8000Hz frequency range
DC voltage can vary between 18V and 27V, depending on the state and level of charge in the battery
The speakers would have some sort of amplifier in them, which would demodulate the audio signal, amplify it and put through the speakers. So they're technically active speakers.
Since this is a hobby project, the wire runs/lengths aren't too big. I have no idea how big they'd be but if I can do this over 50 meters, that'd be perfect.
EDIT: The modulated signal should be around 1Vpp.

I was thinking of using AM to modulate the audio, but I'm not sure if sending lower frequency AM signals over a pair of wires has the same "interference effect" as AM radios.

So.. My question is, what modulation scheme is best for sending audio signals over a pair of low voltage DC power lines?

 

Welcome to AAC.

Since the audio is AC, it can simply be superimposed on the DC. Blocking capacitors can be used to prevent the DC from getting into the audio circuit(s). This is commonly done to power electret microphones. Check out phantom power to see practical circuits.

You really shouldn’t have to do anything particularly sophisticated.

 

The amplifier will then superimpose a signal on the DC power which is the audio signal full-wave rectified, due to the current in the load. Sorting that out from the intended signal is going to be tricky.
You might stand a better chance FM modulating the audio as in one of the data over mains wiring techniques.
Secondly, the standards for voice evacuation systems are complex and need all sorts of things such as the ability to prove that the loudspeaker is actually working. Most such systems use 100V line.

 

The amount of aggravation, engineering, and expense, makes this proposal into an absurdity.

A standard ~70V, or "100-Volt-Line" Transformer-System is tried and true, and most importantly,
really cheap, "rocks and sticks"-reliable, and, even being over ~100-years old, is still readily available,
and super-easy to implement, and allows the use of small-gauge-Wire, even on extremely long runs.

It's also classified as "Type-II" Wiring, just like Computer-Network-Wiring,
meaning that it can be run almost anywhere in a building, except in an Elevator-Shaft, or inside Fire-Stairs.

PA-Speakers are readily available with the appropriate 100V-Line-Transformers pre-installed by the manufacturer.

Two other notes ...........

1)
Forget about ~40Hz low-frequency response, that would be nothing but a gross waste of Power.
~200Hz will be perfectly fine for a Voice-Annunciation-System,
and may even reduce Distortion and increase Fidelity,
and allow for substantially increased Power-Handling if needed.
Everything below, and above, the range of ~200Hz to ~8kHz, should be filtered-out in any case.
This applies to the Original-Voice-Recording too.

2)
Get the Voice-Recording professionally produced by
a Recording-Studio who can add rather severe Dynamic-Range-Compression and Peak-Clipping,
or learn how to do it yourself.
( warning, there's a rather steep-learning-curve, it takes years of experience to do it well )
.
.
.

 

What has not been mentioned at all is what sort of audio. For announcements and speech messages the scheme LQC has described will work OK.
BUT if the goal is to send MUSIC from some source to a set of external amplifiers then it gets more challenging. Construction projects for FM over power wiring are available, both AM and FM at different levels of signal quality. PCB artwork included, found in the "Schematics for free" website. But those circuits are nothing simple, lots of parts and a whole lot of actual electronic stuff involved.
The big thing is that we really do not know enough to make a GOOD suggestion. How much DC power and for what purpose? I can imagine a connection from a car in a parking lot to a gathering in a picnic area with some serious power amplifiers to assure that everybody within a quarter mile can hear the music clearly. I can also imagine very "audiophile satisfying" music in some fancy garden of some huge estate, for formal social gatherings.
Or speakers behind the crowd at an outdoor concert in a city park.
So the amount and purpose of the DC power is quite an important variable. It might even be for a purpose not related to the audio. consider also that there are a few schemes for sending DC power over lines also used for assorted signals presently in use.
[edit] OOPS!! it has been announced that it is to be voice announcements sent to powered speakers, although that is much more a music thing. But most amplfied speakers us AC power and just "line level" audio signals. Audio over amplifier power lines is rarely done, I have not seen it at any higher power levels.

 

What do you mean by "a hobby fire alarm system"?
Do you mean a DIY audio annunciator?

The simple solution is to wire multiple speakers in series/parallel combination.
Or simply, four 4-ohm speakers in series would give a 16-ohm load that most amplifiers can drive.